5 L ARGE -S CALE I MPLEMENTATION
5.5 Practical implementation
The focus in this study has been on the potential for utilizing buildings as term TES and what benefits that can bring to a DH system. If buildings as short-term TES are to be implemented, there needs to be a business model and a method to control the TES. This can be solved in many ways that can be grouped in two categories: direct control and indirect control.
Direct control
With a direct control system, the heat supplier has direct control over the heat load in the utilized buildings. Such solutions can be simple to implement since the buildings can use the already existing control system for the radiator system and the TES control can be implemented by adjusting Δu. A data connection or some other method for the heat supplier to adjust Δu is required for such a solution.
Even if the technical solution is fairly simple, there are organizational obstacles to overcome. The heat supplier and the building owner need to have a contract that covers several areas:
• How large adjustments can be made to the heat load
• How often the heat load can be adjusted
• Who is responsible if there is a problem with the indoor climate in the building
• How the building owner is compensated.
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The first three of these points can be overcome if the indoor temperature is continuously measured and implemented in the control, or if there at least is a fairly accurate model of the building’s thermal properties. But such measures come with a cost and will impact the profitability of short-term TES. It might be more beneficial to have more narrow adjustments to Δu on a larger amount of buildings and avoid implementing the suggested measures.
The last point about how the building owner is compensated can be solved in several ways. Economic remuneration in the form of fixed compensation or reduced heat prices is possible. It is also possible to compensate the building owner for each time the building is used as short-term TES.
Indirect control
With indirect control of building TES, the building’s owner makes adjustments to the heat usage, resulting in a more favorable heat load profile. Some kind of incentive from the heat supplier is required for the building’s owner to make these adjustments. The incentive can be a price model that makes it favorable for the building to use heat when it is favorable from a system perspective. A favorable solution would be to have hourly pricing of heat based on what type of heat generation is on the margin. This would give building owners an incentive to use heat when it is generated in the most economically favorable way, which is often the most efficient and environmentally friendly way.
A solution with hourly heat pricing circumvents the organizational obstacles associated with a direct control approach. The interaction between the heat supplier and the building owners is simple, and all control in the buildings is voluntary and the responsibility of the building’s owner. The obstacles to
overcome are more technical with this strategy. The heat distributer needs to have a model (and preferably a forecast) for the cost of heat generation and some way of communicating the hourly heat prices to the consumers. The consumer, on the other hand, needs to have a control system that can take the cost of heat and the thermal inertia of the buildings into account. This could open a new market section in building automation systems or expand the existing market section for weather forecast control. The infrastructure to implement hourly heat pricing is already in place in buildings with weather forecast control since there is already a model for the building’s thermal properties and an Internet connection that can receive weather forecasts. It should be cost effective in these systems to
supplement them with hourly heat prices and change the target function to be minimized from total energy usage to total energy cost.
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6 C ONCLUSIONS
The pilot test in this study has shown that heavy buildings with a structural core of concrete can tolerate relatively large variations in heat deliveries and still maintain a good indoor climate. Storing 0.1 kWh/m2 floor area of heat will very rarely cause variations in indoor temperature larger than ±0.5°C in the heavy buildings most sensitive to temperature variation. This corresponds to adjusting the outdoor temperature signal, Δu, by 7°C over 9 h. Most heavy buildings will have even smaller variations in indoor temperature and could possibly be used to store larger quantities of energy than the more sensitive buildings.
Both degree hours and stored heat per floor area can be used as tools to estimate the storage capacity limitation and power limitation of buildings or areas that can potentially be used as short-term TES. Which one of these parameters is more favorable depends on what data are available. The third alternative tool, time constant, is less suitable as a measurement for said purpose.
The greatest uncertainty from the pilot test is the small number of tested buildings.
The buildings had a fairly large spread in the variation in indoor temperature, while the spread in stored heat was much more consistent. This is why the “safe side” assumption for a larger scale TES is based on the most sensitive of the heavy buildings in the pilot test. If a building’s short-term TES on a large scale is to be implemented, it would involve many buildings. Such a solution would be fairly costly if many buildings need to be extensively tested. The results from this pilot test can be used in similar buildings without much testing since the indoor temperature variation will be well within acceptable values in most buildings.
Variation in heat loads in DH systems can be greatly reduced by utilizing buildings as short-term TES. Using about 500 substations for short-term TES in large residential buildings would provide a capacity for storing heat equivalent to constructing a hot water storage tank with a volume of 14,200 m3 for the city of Gothenburg. The yearly heating energy load for these 500 substations corresponds to 20% of the heat generation. Such a TES could decrease the daily variations in heat load by 50%, reduce the need for peak heat generation, and reduce the
number of starts and stops of heat generation units. This can result in reduced heat generation in oil and gas HOBs in Gothenburg by 10% to 20%. Assuming that the required adjustments to the substations can be made cheaper than 6,000€ to 12,000€ per substation, utilizing buildings as short-term TES can be a more economical alternative than hot water storage tanks.
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